Mast cell tumour, a common skin tumour of dogs, was treated effectively with radiation in 85 dogs. Prognosis for tumour-free and survival times were affected by several factors. Dogs with mast cell tumour of the extremities and dogs with clinical stage 0 (no gross evidence of disease), had significantly longer tumour-free times than did dogs with mast cell tumours of the trunk and dogs with stages II and III neoplasms. Survival times also were influenced by neoplasm location, clinical stage, and neoplasm grade. Dogs with grade II mast cell tumours lived longer than did dogs with grade III mast cell tumours. Overall tumour-free and survival rates were 78.8 and 77% and 76.2 and 73.2% at 1 and 2 years, respectively. At 1 year, the rate of metastasis was higher for dogs with grade III neoplasms, for dogs with neoplasms on the trunk, and for dogs with clinical stages II and III neoplasms..